This invention relates generally to archery. More specifically, the invention relates to compound bows; and most specifically, the invention relates to improve pulley assemblies for use in compound bows.
In a compound archery bow, the force required to move the bow string (i.e. the draw force) varies as a function of the draw length. In a typical compound bow, the draw force is initially fairly high, and as the bow approaches a fully drawn condition, the force decreases. This xe2x80x9clet offxe2x80x9d in draw weight permits an archer to hold a fully drawn bow in a relatively steady position thereby providing an increase in accuracy. As a result of the let off and the force-draw characteristics of the bow, the amount of energy that can be stored is maximized thereby providing for a flatter path of travel of the arrow, higher arrow velocity, and an increase in the amount of energy delivered to the target. For these reasons compound bows are widely used by target shooters and hunters.
A typical compound bow normally includes two resilient limbs mounted upon a handle, two cable sections, a bow string and two pulley assemblies pivotally mounted at the tips of the limbs. The cable sections operate to compress the limbs in a controlled manner as the bow string is drawn, and the pulleys rotate to feed out and/or take up the cable and bow string. An arrangement of cams or levers generally disposed at the tips of the limbs of the bow, and operating through the cables, is employed to give a mechanical advantage as the bow is drawn and thereby modify the force draw curve.
An early compound bow design is disclosed in U.S. Pat. No. 3,486,495. The bow disclosed in this patent includes a cam/pulley arrangement at each end of the bow limb, and as such is referred to as a dual cam bow. In a typical dual cam design, one end of the bow cable is terminated and attached to a bow limb while the other ends wrap up on a groove in a pulley assembly, disposed upon the other limb, as the bow string is drawn. Difficulties arise in synchronizing the action of the two cams in dual cam bows of this type, and when the cams are out of synchrony, erratic arrow travel, and hence inaccuracies arise. While dual cam bows are still being utilized, the art has been turning to what is termed single cam bows. In bows of this type, the force multiplying cam action is provided by a single cam wheel/pulley assembly associated with one of the bow limbs. The other limb supports a pulley assembly which is referred to as an idler pulley, and this pulley does not exert any camming action. Use of a single cam bow eliminates problems of cam timing; however, problems still occur because the non-symmetrical nature of the bow performance causes the nock point of the bow string (that is to say the point on the bow string at which an arrow is supported) to travel in a non-linear path. This deviation from linearity adversely affects the flight characteristics of an arrow.
In response to problems of the early single cam bows, various hybrid designs have been developed in which the non-camming pulley operates to compensate for, and linearize, at least to some degree, nock point travel. One such hybrid bow assembly is disclosed in U.S. Pat. No. 5,505,185. In both single cam designs and hybrid designs, one of the cables operates in a manner similar to that of a dual cam bow; that is to say, one end of the bow cable is terminated and attached to a bow limb (or in some instances a pulley assembly), while the other end wraps up on a groove of a pulley assembly associated with the other limb, while the bow string is drawn. In the single cam and hybrid designs, the second pulley assembly operates to unwrap or feed out a cable from a groove in the pulley assembly while the bow string is drawn. In this manner, cable is fed from one pulley assembly and wound onto the other pulley assembly when the bow is drawn.
While hybrid designs provide performance which is superior to that of dual cam bows and conventional single cam bows, the pulley assemblies thereof are relatively complicated, and tend to be large and high in mass. This can cause problems of noise and vibration as well as increase the mass of the bow. All of these factors adversely affect performance. Therefore, it will be appreciated that there is still a need for improved compound bow assemblies which are simpler, lighter in weight, and easier to operate and maintain than are prior art bows. As will be appreciated from the drawings, discussion and description hereinbelow, the present invention provides an improved pulley assembly for compound bows. The pulley assembly of the present invention operates in a manner which is novel and distinct from prior art compound bow pulleys. The pulley of the present invention permits the manufacture of compound bows which are light in weight, relatively quiet, powerful and easy to maintain and use. The pulley assembly of the present invention may be employed in a number of bow designs and configurations as will be described hereinbelow.
There is disclosed herein a pulley assembly for a compound bow. The pulley assembly comprises an axle journal which defines a pivot axis about which said pulley assembly is rotatable. The assembly further includes a pulley body. The pulley body includes a first, a second and a third groove defined therein. Each groove describes a curve which extends at least partway around the pivot axis. The first groove is a bow string groove operative to receive a portion of a bow string therein, the second groove is to receive a portion of a first bow cable section therein and the third groove is operative to receive a portion of a second bow cable section therein. The pulley assembly is configured and operative so that when it is incorporated into a compound bow so as to be rotatable about the pivot axis, and when the bow string is unwound from the first groove, a portion of said first bow cable section is wound into the second groove and said portion of said second bow cable section is wound into the third groove. The radii of one or more of the first, second and third curves may be constant or variable. In specific embodiments, when the bow string is unwound from the first groove, the rate at which the first bow cable section is wound into the second groove differs from the rate at which the second bow cable section is wound into the third groove. The pulley body may be a unitary body, or may be comprised of a plurality of subparts which are joined together. In such instance, the subparts may be positionally adjustable relative to one another.
The present invention also includes a compound bow which includes at least one of the pulley assemblies of the present invention.